[unified-consent] Add 'Cancel sync?' dialog

- Add sync setup cancel dialog for confirming
  the sync cancellation during opt-in.
- This dialog is shown when the user clicks
  the 'Cancel' button in the account control
  or navigates away from the sync page.

Screenshot:
https://drive.google.com/file/d/1TaQbBntf1Or4y4CPOzFDHhKf9qLZWpDI/view?usp=sharing

Bug: 919745
Change-Id: I88de095ce7947cabf9363208579556a6486283a3
Reviewed-on: https://chromium-review.googlesource.com/c/1430016
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#626780}(cherry picked from commit 501c7b5248048022035d30d36f988b2fc32539bf)
Reviewed-on: https://chromium-review.googlesource.com/c/1449662
Reviewed-by: Thomas Tangl <tangltom@chromium.org>
Cr-Commit-Position: refs/branch-heads/3683@{#118}
Cr-Branched-From: e51029943e0a38dd794b73caaf6373d5496ae783-refs/heads/master@{#625896}
5 files changed